Compositions and methods for inhibiting tannin release

ABSTRACT

The invention relates to compositions and methods whereby tannin staining of a natural or synthetic lumber product can be inhibited or prevented. The compositions and methods described herein can be used in conjunction with compositions and methods for inhibiting microbial growth on or in lumber products.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

REFERENCE TO A SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING APPENDIX SUBMITTED ON A COMPACT DISC

[0003] Not applicable.

BACKGROUND OF THE INVENTION

[0004] The invention relates generally to the field of reducing tannin staining of cellulose-containing materials. Tannins are a group of polyphenolic compounds that occur in the vascular tissues of terrestrial plants.

[0005] Synthetic lumber has been used as a substitute for wood in areas where wood can deteriorate quickly due to environmental conditions. Although in the past, its commercialization was limited by costs, modern recycling techniques and low cost extrusion manufacturing capability have permitted greater penetration by polymer-fiber composite materials into the commercial and residential markets. One such product manufactured under the trademark TREX, by Trex Company, LLC, Winchester, Va., consists of a polyethylene-wood fiber blend which is extruded into board dimensions for decking applications. Polyethylene-wood composite boards in {fraction (5/4)} inch thicknesses have sufficient rigidity to be used as decking planks, but typically are not recommended for structural wood substitutes, such as the lattice structure often used as a support for decks.

[0006] Many types of synthetic lumber comprise cellulose fibers derived from terrestrial plants. For examples, synthetic lumber products comprising wood flour or paper fibers are known. The cellulosic materials can contain tannins. Natural wood products also contain tannins. Under certain environmental conditions, tannins that are present in the cellulosic materials of natural or synthetic lumber products can leach to the surface of the product and be deposited there. Such deposition is considered unsightly and undesirable for many applications, particularly where the appearance of the lumber product is important. This problem is recognized in the art, and is referred to as tannin staining.

[0007] Prior art methods of inhibiting tannin staining involve sealing a lumber product in such a way that water cannot reach the cellulosic materials of the product. If water does not contact the cellulosic materials, leaching of tannins therefrom and deposition on the product surface are prevented. Numerous primer, paint, and sealing products are commercially available. Such products are suitable for protecting lumber products from rain and humidity, but can crack, peel, or degrade over time. Application of such products to lumber products can also inhibit application of other desired products, such as finishing paints or stains.

[0008] Growth of molds and fungi (“microbial growth”) on and in lumber products can detract from their appearance, their strength, and their ability to bind appropriately with finishing products such as paints, primers, stains, and sealers. Microbial growth can occur under conditions in which lumber products are ordinarily stored. Furthermore, natural wood products and some synthetic lumber products contain compounds that can be metabolized by microbes, further enhancing the rate and extent of microbial growth on those products. For example, calcium carbonate is sometimes used as a filler in synthetic lumber products, but can be metabolized by many molds. For this reason, mold growth on calcium carbonate-containing synthetic lumber products can be a particular problem.

BRIEF SUMMARY OF THE INVENTION

[0009] The invention relates to a method of treating a substrate that comprises cellulose and a tannin. The method comprises contacting a surface of the substrate with an aqueous composition comprising an agent that binds covalently with the tannin. The pH of the composition should not be greater than 9.0, nor less than 3.0. Preferably, the pH of the composition is in the range 4.0 to 8.5, 5.0 to 8.0, or 6.0 to 7.5. A preferred pH range is 6.5 to 7.2. Compositions having a pH of substantially 7 are suitable.

[0010] In one embodiment of this method, the antimicrobial agent is a compound that has the chemical formula X—L—Tbm. In this formula, X is a chemical moiety that exhibits antimicrobial activity, L is an optional linking moiety, and Tbm is a tannin-binding moiety. For example, X can a quaternary ammonium moiety, such as one having the chemical formula ((R¹)₃—N⁺)—. In this formula, each R¹ can be independently selected from the group consisting of —H and branched and straight-chain C₁ to C₂₄ alkyl radicals. L can, for example, be a branched or straight-chain C₁ to C₂₄ alkyl diradical, optionally having one or more —O— or —N— diradicals in its backbone. Tbm can be selected from the group consisting of a silane moiety, and an alkoxysilyl moiety, although it can also be other tannin-binding moieties.

[0011] In another embodiment of the methods described herein, the antimicrobial agent is selected from the group consisting of an octadecylaminodimethyltrihydroxysilylpropyl ammonium salt and an octadecylaminodimethyltrimethoxysilylpropyl ammonium salt.

[0012] The composition used in the methods described herein can comprise the conjugate base of an organic acid. Suitable organic acids include dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, fumaric acid, glutaric acid, adipic acid, and maleic acid. Owing to its known utility in certain applications, oxalic acid is suitable for decking applications, for example.

[0013] The methods described herein are useful for treating a variety of substrates. Examples of suitable substrates include polymer—cellulose composites such as those comprising polyvinylchloride. Wood and other exterior building materials are also suitable substrates. The methods are useful for a variety of purposes, such as treating planks and other decking materials.

BRIEF SUMMARY OF THE SEVERAL VIEWS OF THE DRAWINGS

[0014] Not applicable.

DETAILED DESCRIPTION OF THE INVENTION

[0015] The invention relates to the discovery that tannin staining of lumber products can be reduced during application of water or aqueous compositions by avoiding extremes of pH in the water or composition during its application. Furthermore, prolonged inhibition or prevention of tannin staining and prevention of microbial growth can be achieved with lumber products by treating such a product with a composition comprising an aqueous composition of an antimicrobial agent that covalently binds with a tannin in the product at a relatively neutral pH.

Definitions

[0016] An antimicrobial agent is a compound that inhibits or prevents growth or replication of a mold or fungus or that kills a mold or fungus.

[0017] Lumber products include natural wood products (e.g., whole cut wood), manufactured wood products (e.g., plywoods, chip boards, and particle boards), and polymer-cellulose fiber composites (e.g., composites of polyvinylchloride or polyethylene with wood flour or paper fiber).

Detailed Description

[0018] It has been discovered that water-based leaching of tannins from a natural or synthetic lumber product can be reduced, inhibited, or prevented by maintaining the pH of aqueous compositions that contact the lumber product relatively neutral. Advantageously, both microbial growth and tannin staining can be inhibited by treating a lumber product with a tannin-binding antimicrobial agent at a relatively neutral pH.

pH

[0019] It has been discovered that the pH of compositions with which substrates that comprise both cellulose and one or more tannins are treated is important for determining the rate, extent, or both, of subsequent tannin release from the composition. Tannins have been discovered to be more soluble in very alkaline aqueous systems than in less aqueous systems. For this reason, the pH of any aqueous solution used to treat a lumber product should be maintained not greater than about 9.0, and preferably not greater than 8.5, 8.0, or 7.5. Tannins have also been discovered to be more soluble in very acidic aqueous systems than in more neutral systems. For this reason the pH of any aqueous solution used to treat a lumber product should be maintained not less than 3.0, and preferably not less than 4.0, 5.0, or 6.0. An optimal pH range for aqueous solutions used to treat lumber products is 6.5 to 7.2. Other suitable ranges of pH include 6.0 to 7.5, 5.0 to 8.0, and 4.0 to 8.5, although the more neutral pH ranges are preferable. In one embodiment, the pH of any aqueous solution used to treat a lumber product is maintained at about 7.0.

[0020] The way in which the pH of any aqueous solution with which a lumber product is treated is not critical. A simple way of adjusting the pH of such a solution is to add an acid or base to the solution. The identity of the acid or base is not critical. Simple inorganic acids and bases (e.g., HCl and NaOH) can be appropriate. However, certain acids or bases or types of acids or bases can be preferred for substrates to be used in various applications. When human contact with the treated substrate is anticipated, the acid or base should be selected so as to minimize toxicity and irritation towards humans and to improve appearance. For example, if a synthetic lumber product is to be treated as described herein and thereafter used in a residential decking application, and if the treatment involves contacting the product with an antimicrobial composition that is ordinarily very alkaline, then the pH of the antimicrobial composition can be made more nearly neutral by adding oxalic acid (a compound known to be acceptable in residential decking applications) thereto.

Tannin-Binding Agents

[0021] Inhibition of tannin release from a substrate that comprises cellulose and the tannin can be achieved by increasing the molecular weight of the tannin, by increasing the hydrophobicity of the tannin, or both. The molecular weight of a tannin can be increased by attaching a moiety to the tannin (e.g., attaching a silane or siloxane compound thereto) or by crosslinking the tannin with another tannin or with another component of the substrate (e.g., with cellulose in the substrate). Regardless of the reagent used to derivatize or cross-link the tannin, the reagent should be used in an aqueous formulation having a relatively neutral pH as described herein.

[0022] The important characteristics of the tannin-binding agents used as described herein depend on the end use of the substrate to be treated therewith. Many lumber products are used in construction applications in which fire resistance and relative non-toxicity upon incineration are important characteristics. In such applications, tannin-binding agents can be selected to maximize fire resistance and to minimize toxicity upon incineration. In applications in which human contact with the treated substrate is anticipated (e.g., use of treated synthetic lumber products in residential decks, rails, and siding), the tannin-binding agent should be selected so that the treated product exhibits low toxicity and does not exhibit undesirable side effects (e.g., pain, burning, or discoloration) with animal tissues.

[0023] Numerous silane and siloxane compounds are known in the art, and substantially any of these can be used to bind with or cross-link a tannin. Examples of such compounds include silanes, alkoxysilanes, chlorosilanes, and other derivatized silanes offered for sale by companies such as Dow Coming and others. Silane and siloxane compounds can be used alone or in conjunction with other compounds.

[0024] Diacids and diacid anhydrides can be used to cross-link tannins, as described in U.S. Patent Application publication number 20030004232. Examples of suitable diacids include oxalic acid, malonic acid, succinic acid, fumaric acid, glutaric acid, adipic acid, and maleic acid. Oxalic acid is well known for use in cleaning and brightening residential lumber products, and its relative non-toxicity and suitability for residential applications recommend its use for substrates with which human contact is anticipated.

[0025] A preferred tannin-binding agent is an anti-microbial agent that binds covalently with one or more tannins in the substrate. Such an agent both increases the molecular weight of the tannin (i.e., thereby decreasing its proclivity to leach from the substrate) and inhibits microbial growth on the substrate, within the substrate, or both. Such an agent will often be a molecule wherein one part of the molecule exhibits antimicrobial activity and a different part of the molecule is capable of binding covalently with the tannin(s) in the substrate. The two parts of the molecule can be separated by a linker. If one or more of the linker, the antimicrobial part, or the tannin-binding part of the molecule exhibits significant hydrophobicity, then the hydrophobic nature of the agent can further inhibit tannin release from the substrate.

[0026] In one embodiment, the tannin-binding agent has the following structure.

[0027] X—L—Tbm

[0028] In this structure, X is a chemical moiety that exhibits antimicrobial activity, L is an optional linking moiety, and Tbm is a tannin-binding moiety.

[0029] Many chemical moieties that exhibit antimicrobial activity are known, and X can be substantially any of them. Preferably, X does not exhibit significant adverse effects when external human tissues contact it. Suitable X moieties include quaternary ammonium moieties such as those having the structure ((R¹)₃—N⁺)—, wherein each R¹is independently selected from the group consisting of —H and branched and straight-chain C₁ to C₂₄ alkyl radicals. In one embodiment, at least one of the R¹ moieties is a C₁₂ to C₂₄ alkyl radical, such as a C₁₈ alkyl radical (i.e., a CH₃(CH₂)₁₇-radical).

[0030] The R¹ moieties can, optionally, be substituted with one or more hydroxyl or primary, secondary, or tertiary amine groups, and the backbone of the radicals can have one or more —O— or —N— diradicals therein. Furthermore, the charged nitrogen atom can be part of a ring system, preferably an unsaturated ring system. The antimicrobial properties of quaternary ammonium salts are known in the art, and substantially any such salt can be adapted for use herein simply by connecting it with the linker (if present) and the tannin-binding moiety. The identity counter ion, if any, used with the quaternary ammonium moiety is not critical. Standard counter ions (e.g., chloride or acetate) can be used.

[0031] When the linking moiety is present, its identity is not critical. The X moiety can be linked directly with the Tbm moiety. Alternatively, a short (e.g., C₁ to C₆), long (e.g., C₁₂ to C₂₄), or intermediate (e.g., C₆ to C₁₂) alkyl radical can be interposed between X and Tbm. The linker can be flexible (e.g., a straight chain alkyl radical), relatively inflexible (e.g., a hydrocarbon backbone having one or more unsaturated bonds), or rigid (e.g., an unsaturated ring system).

[0032] The identity of the Tbm moiety is not critical, so long as the moiety is able to bind covalently with one or more functional group present in a tannin of the substrate to be treated. Suitable examples of Tbm moieties include silane and siloxane moieties and carboxylic acid moieties. Silane moieties occur in many known wood-treatment products, and the Tbm moiety can replicate the silane moiety of substantially any of those products. Silane moieties having one or more alkoxy substituents (i.e., alkoxysilyl moieties) are also suitable.

[0033] The tannin-binding agents described herein are either commercially available or can be synthesized by ordinary chemical synthetic methods based on known compounds.

Treated Substrate

[0034] The compositions and methods described herein

[0035] Tannins occur in many sources of cellulose fibers , including recycled paper products, such as agriculturally-derived fibers, pulp, newsprint, soft woods, such as pine, or hard woods from deciduous trees. Hard woods are generally preferred for incorporation into synthetic lumber products because they absorb less moisture. Although hard wood is the primary source of fiber for synthetic lumber products, additional fiber make-up can be derived from a number of secondary sources including soft wood fibers, natural fibers including bamboo, rice, sugar cane, and recycled or reclaimed fiber from newspapers, cardboard boxes, computer printouts, etc. The synthetic lumber products can also be made using wood flour of about 10-100 mesh, preferably 20-30 mesh.

[0036] The observations made herein relating to the pH of aqueous compositions used to treat cellulose-containing substrates are not limited to lumber products. The observations are equally applicable to aqueous compositions contacted with substantially any material that contains both cellulose and one or more tannins. By way of examples, such materials include finished wood products, paper products, leather products, food products, and the like.

Treatment of Substrates

[0037] Many known methods of treating wood or another cellulose- and tannin-containing substrate result in tannin staining, either upon performance of the method or thereafter upon subsequent wetting of the treated substrate. It has been discovered that tannin staining that occurs during and after such treatments can be reduced if the treatment is normally performed at a relatively extreme pH. Release of tannin from the substrate is reduced by making the pH of any aqueous reagent(s) used in the treatment more neutral. The pH of the reagent(s) is made more neutral by adding acid (to reduce pH) or base (to increase pH) to the reagent(s) prior to application of the reagent(s) to the substrate. Suitable pH values and suitable pH-adjusting agents are described herein.

[0038] The method by which a pH-adjusted reagent is applied to the substrate is not important. Reagents can be applied using and known method or equipment for applying liquids to solid substrates. For example, brushes, rollers, spray apparatus, foggers, dunking or dipping tanks, and the like can be used to apply a liquid to a substrate.

Foamed Polymer-Fiber Composite Lumber

[0039] In an important embodiment, the substrate that is treated as described herein is a foamed polymer-fiber composite, such as one described in U.S. Pat. No. 6,344,268. Such composites generally contain about 35-75 wt. % resinous materials, such as thermoplastic and thermosetting resins. Examples of suitable materials include PVC, polyethylene, polypropylene, nylon, polyesters, polysulfones, polyphenylene oxide and sulfide, epoxies, cellulosics, and the like. A preferred thermoplastic material for the is PVC.

[0040] PVC thermoplastics comprise the largest volume of thermoplastic polymers in commercial use. PVC used in synthetic lumber products can be combined with impact modifiers, thermal stabilizers, lubricants, plasticizers, organic and inorganic pigments, fillers, biocides, processing aids, flame retardants, or other commonly available additive materials, as is known. PVC can also be combined with other vinyl monomers in the manufacture of polyvinyl chloride copolymers. Such copolymers can be linear copolymers, graft copolymers, random copolymers, regular repeating copolymers, block copolymers, or the like. Examples of monomers that can be combined with vinyl chloride to form vinyl chloride copolymers include acrylonitrile; alpha-olefins such as ethylene and propylene; chlorinated monomers such as vinylidene dichloride; acrylate monomers such as acrylic acid, methylacrylate, methyl-methacrylate, acrylamide, and hydroxethyl acrylate; styrenic monomers such as styrene, alpha methyl styrene, and vinyl toluene; vinyl acetate; and other commonly available ethylenically. unsaturated monomer compositions. Such monomers can be used in an amount of up to about 50 mole %, the balance being vinyl chloride. PVCs can be compounded to be flexible or rigid, tough or strong, to have high or low density, or to have any of a wide spectrum of physical properties or processing characteristics, as is known in the art. PVC resins can also be alloyed with other polymers, such as ABS, acrylic, polyurethane, and nitrile rubber to improve impact resistance, tear strength, resilience, or proccessability. They can be produced water-white in either rigid or flexible compositions, or they can be pigmented to almost any color.

[0041] In the preferred embodiments of this invention, rigid PVC, optionally containing a small amount of plasticizer, is employed. This material is a hard and tough and can be compounded to have a wide range of properties, including impact resistance and weatherability, e.g., fading color to a wood grey appearance. It also has a tensile strength of about 6,000-7,500 psi, a percent elongation of about 40-80%, and a tensile modulus of about 3.5-6.0×10⁶ psi. It can be acceptably used without chlorination, to about 140° F., and with chlorination to about 220° F. It also has a coefficient of thermal expansion of about 3-6×10⁵ inch/inch-° F.

[0042] PVC composite building materials of this invention can be injection or vacuum molded, extruded and drawn, using customary manufacturing techniques for thermoplastic and thermosetting materials. In one embodiment, a mixture of PVC regrind or virgin compound is compounded and then heated and extruded through a die to produce boards and other shapes having a length of about 4-20 feet and thicknesses ranging from 0.05-6.0 inches. The extruded thermoplastic boards can be subject to further molding, calendaring, and finishing to provide a wood grain or fanciful texture.

[0043] Synthetic lumber products can contain cellulose fiber, such as fiber derived from wood (e.g., red oak powder) or cotton. When present, the cellulosic material preferably comprises about 25-60 wt. % of the final lumber product. The methods described herein for inhibiting tannin release from the cellulosic material are particularly applicable when materials having a relatively high tannin content are used. Many woods (e.g., cedar, redwood) are known to have a relatively high tannin content, while cotton products generally contain much less tannins. Additionally, inorganic fillers, such as calcium carbonate, talc, silica, etc. can be incorporated into the synthetic lumber product.

[0044] Foamed polymer-fiber composite synthetic lumber products are made by combining a resin (e.g., PVC) and a cellulosic material (e.g., wood flour) with a chemical blowing agent, or by introducing a gaseous medium into a molten mixture of the resin and cellulosic material to produce a series of trapped bubbles prior to thermo-forming the mixture, for example, by molding, extrusion or co-extrusion. Such processes for making rigid foam articles are generally known.

[0045] Chemical blowing agents can be any of a variety of chemicals which release a gas upon thermal decomposition. Chemical blowing agents may also be referred to as foaming agents. The blowing agent, or agents, if more than one is used, can be selected from chemicals containing decomposable groups such as azo, N-nitroso, carboxylate, carbonate, hetero-cyclic nitrogen-containing and sulfonyl hydrazide groups. Generally, they are solid materials that liberate gas when heated by means of a chemical reaction or upon decomposition. Representative compounds include azodicarbonamide, bicarbonates, dinitrosopentamethylene tetramethylene tetramine, p,p′-oxy-bis (benzenesulfonyl)-hydrazide, benzene-1,3-disulfonyl hydrazide, aso-bis-(isobutyronitrile), biuret, and urea.

[0046] The blowing agent can be added to the polymer in several different ways which are known to those skilled in the art, for example, by adding the solid power, liquid or gaseous agents directly to the resin in the extruder while the resin is in the molten state to obtain uniform dispersion of the agent in the molten plastic. Preferably the blowing agent is added before the extrusion process and is in the form of a solid. The temperature and pressure to which the foamable composition of the invention are subjected to provide a foamed composition will vary within a wide range, depending upon the amount and type of the foaming agent, resin, and cellulosic fiber that is used. Preferred foaming agents are selected from endothermic and exothermic varieties, such as dinitrosopentamethylene tetramine, p-toluene sulfonyl semicarbazide, 5-phenyltetrazole, calcium oxalate, trihydrazino-s-triazine, 5-phenyl-3,6-dihydro-1,3,4-oxandiazin-2-one, 3,6-dihydro 5,6-diphenyl-1,3,4 oxadiazin-2-one, azodicarbonamide, sodium bicarbonate, and mixtures thereof.

[0047] In addition to the above, a coloring agent can be added to the compounded mixture, such as dyes, colored pigments, or fly ash, or a mixture of these ingredients depending on the resulting color, and cost considerations. Such additives can provide “weatherability” or a faded grayish coloring or a permanent tint, such as blue, green or brown. This invention can be further understood by reference to the following examples.

[0048] After a synthetic lumber product is formed, it can be treated using a composition described herein to inhibit microbial growth thereon and to prevent or inhibit tannin staining of the product if it is exposed to moisture.

[0049] The disclosure of every patent, patent application, and publication cited herein is hereby incorporated herein by reference in its entirety.

[0050] While this invention has been disclosed with reference to specific embodiments, it is apparent that other embodiments and variations of this invention can be devised by others skilled in the art without departing from the true spirit and scope of the invention. The appended claims include all such embodiments and equivalent variations. 

What is claimed is:
 1. A method of treating a substrate that comprises cellulose and a tannin, the method comprising contacting a surface of the substrate with an aqueous composition comprising an agent that binds covalently with the tannin, wherein the pH of the composition is not greater than 9.0.
 2. The method of claim 1, wherein the pH of the composition is not less than 3.0.
 3. The method of claim 1, wherein the pH of the composition is in the range 4.0 to 8.5.
 4. The method of claim 1, wherein the pH of the composition is in the range 5.0 to 8.0.
 5. The method of claim 1, wherein the pH of the composition is in the range 6.0 to 7.5.
 6. The method of claim 1, wherein the pH of the composition is in the range 6.5 to 7.2.
 7. The method of claim 1, wherein the pH of the composition is substantially
 7. 8. The method of claim 1, wherein the antimicrobial agent is a compound that has the chemical formula X—L—Tbm, wherein X is a chemical moiety that exhibits antimicrobial activity, L is an optional linking moiety, and Tbm is a tannin-binding moiety.
 9. The method of claim 8, wherein X is a quaternary ammonium moiety.
 10. The method of claim 9, wherein Tbm is selected from the group consisting of a silane moiety, and an alkoxysilyl moiety.
 11. The method of claim 10, wherein X has the chemical formula ((R¹)₃—N⁺)— wherein each R¹ is independently selected from the group consisting of —H and branched and straight-chain C₁ to C₂₄ alkyl radicals.
 12. The method of claim 11, wherein L is a branched or straight-chain C₁ to C₂₄ alkyl diradical, optionally having one or more —O— or —N— diradicals in its backbone.
 13. The method of claim 11, wherein Tbm is selected from the group consisting of a silane moiety, and an alkoxysilyl moiety.
 14. The method of claim 1, wherein the antimicrobial agent is selected from the group consisting of an octadecylaminodimethyltrihydroxysilylpropyl ammonium salt and an octadecylaminodimethyltrimethoxysilylpropyl ammonium salt.
 15. The method of claim 1, wherein the antimicrobial agent is octadecylaminodimethyltrihydroxysilylpropyl ammonium chloride.
 16. The method of claim 1, wherein the composition comprises the conjugate base of an organic acid.
 17. The method of claim 16, wherein the organic acid is a dicarboxylic acid.
 18. The method of claim 17, wherein the organic acid is selected from the group consisting of oxalic acid, malonic acid, succinic acid, fumaric acid, and oxaloacetic acid.
 19. The method of claim 17, wherein the organic acid is oxalic acid.
 20. The method of claim 1, wherein the substrate is a polymer—cellulose composite.
 21. The method of claim 20, wherein the polymer is polyvinylchloride.
 22. The method of claim 1, wherein the substrate is wood.
 23. The method of claim 1, wherein the substrate is an exterior building material.
 24. The method of claim 23, wherein the substrate is a decking material.
 25. The method of claim 24, wherein the decking material is a plank. 